This report describes studies on the biochemical and physiological mechanism by which cell functions are regulated by hormones, agonists, and in pathophysiological states, including aging. The findings summarize investigations on: 1. Age-associated changes in gene expression of the alphai subunits of GTP-binding proteins in rat renal cells. 2. Purification and quantitation of alphai subunits of GTP binding protein in the renal cortex. 3. Adrenergic regulation of cAMP production in OK cells. 4. Desensitization of OK cells to PTH. 5. Relationship of PTH responsiveness in isolated nephron segments with age. 6. Meurotransmitter regulation of cytosolic Ca2+ in bone cells. 7. Regulation of protein kinases C and A in OK cells. 8. PTH are purinergic regulation of cytosolic Ca2+ in OK cells. 9. Age-associated changes in hormonal of cytosolic Ca2+ in proximal tubules in female rats. 10. Purinergic regulation of cytosolic Ca2+ in proximal tubules. 11. Protein kinase C regulation of hormonal responsiveness in proximal tubules.